The present invention relates to cranes, more particularly to methodologies for controlling pendulation that is associated with motion of suspended payloads during operation of cranes, such as rotary boom (slewing pedestal) cranes mounted aboard ships for transferring cargo to piers or other ships.
Crane technology is prevalent in a variety of settings for effecting lift-on, lift-off transfer of cargo. A “gantry crane” implements a horizontally moveable trolley from which a payload is suspended. A “slewing pedestal crane” (also commonly referred to as a “rotary boom crane” or a “rotary jib crane”) involves the suspension of a payload from the tip of a rotatable “boom” (“jib”). According to a “simple” type of slewing pedestal crane, a payload hoist line extends between the boom tip and the payload. The operator of a simple type of slewing pedestal crane is challenged with the task of manually controlling the crane in three degrees-of-freedom, viz., slew (horizontal rotational motion of the boom that results in translation of the payload in a direction transverse to the orientation of the jib), luff (vertical rotational motion of the boom that results in translation of the payload in a direction parallel to the orientation of the jib), and hoist (vertical translation of the payload).
Known in the art is a type of slewing pedestal crane that incorporates a so-called “Rider Block Tagline System” (“RBTS”). An RBTS-equipped crane includes a boom, a rider block (which is situated generally intermediate the boom tip and the payload), a rider block lift line (which extends between the boom tip and the rider block), a payload hoist line (which extends between the boom tip and the payload and is reeved through the rider block), a left tagline, and a right tagline. An RBTS-equipped type of slewing pedestal crane, more complicated than a simple type of slewing pedestal crane, is characterized by the three aforementioned degrees of freedom plus two additional degrees of freedom, viz., the vertical and horizontal positions of the rider block. Due to its greater complexity as compared with a simple slewing pedestal crane, an RBTS-equipped slewing pedestal crane demands greater dexterity and decision-making from the crane operator. Of particular note, the crane operator is required to maintain the rider block within a “feasibility region” in three-dimensional space in order to maintain operability of the RBTS-equipped crane.
The complexity of operating an RBTS-equipped slewing pedestal crane can be alleviated in a manner such as disclosed by Naud et al. U.S. Pat. No. 6,039,193 issued 21 Mar. 2000, entitled “Integrated and Automated Control of a Crane's Rider Block Tagline System,” incorporated herein by reference. Naud et al.'s automatic control of the RBTS is “integrated” with the RBTS-equipped crane so as to, in effect, reduce the number of degrees-of-freedom confronting the crane operator from five degrees-of-freedom to the three degrees-of-freedom that characterize a simple slewing pedestal crane. According to Naud et al., automated control is exercised with respect to the vertical and horizontal positions of the rider block. The method of Naud et al. includes generating a matrix defining incremental changes of the rider block's position in the context of a coordinate system, providing a vector defining velocity criteria for the rider block, multiplying the vector by an inversion of the matrix to obtain a control matrix defining speed and direction of travel for the rider block lift line and the taglines, and controlling movement of the rider block lift line and the taglines using the control matrix.
The RBTS was developed by the U.S. Navy in the mid-1970s to improve the capability of conventional lattice-boom construction cranes for use in container-handling operations in an offshore environment. An important motivation for the U.S. Navy in this regard was to seek to mitigate “pendulation” associated with cargo handling at sea. The principle pendulation-mitigating feature of the RBTS is the presence of a rider block, which serves to effectively reduce the pendulum length to that portion of the hoist line that is between the rider block and the payload. Such pendulum length reduction tends to increase the payload's oscillatory frequency, thereby preventing the entrainment of the payload's oscillation with respect to the oscillation characterizing the ship's motion. Pendulation—swinging or swaying of the payload attached to one or more hoist lines—is a fundamental problem associated with control of a slewing pedestal crane. Crane operators usually seek to avoid or minimize pendulation.
The following paper, which discloses a Pendulation Control System (PCS) for a simple type of ship-based rotary crane, is incorporated herein by reference: Michael Agostini, Gordon G. Parker, Kenneth Groom, Hanspeter Schaub and Rush D. Robinett, “Command Shaping and Closed-Loop Control Interactions for a Ship Crane,” Proceedings of the American Control Conference, Anchorage, Ak., 8-10 May 2002, pages 2298-2304. According to the methodology disclosed by Agostini et al. 2002, a payload mass is conceived to swing on the end of a spherical pendulum that includes a payload hoist line, which is attached to a boom, which is attached to a rotatable column having a geometric axis that is perpendicular to the deck of a ship. The crane has three degrees-of-freedom, viz., slew, luff and hoist. The perpendicular column can be rotated clockwise or counterclockwise; this is referred to as “slewing.” The boom can be rotated to elevate or lower the tip of the boom, thereby positioning the payload closer to or farther from the crane column; this is referred to as “luffing.” The length of the payload hoist line can be lengthened or shortened; this is referred to as “hoisting.” The crane operator positions the payload by issuing luff, slew and hoist commands in real time.
Agostini et al. 2002's control strategy for mitigating pendulation combines three controllers that interact with each other, viz., a command shaper, a ship motion compensator, and a swing damper. The command shaper filters (“shapes”) the operator's commands, preventing the inadvertent addition thereby of energy to the system. The ship motion compensator compensates for sea-induced crane base motion by isolating energy; it prevents transmission of energy from the sea into the payload. An inertial measuring unit can be situated on the ship to measure the sea-induced crane base motion in terms of six degrees-of-freedom, viz., roll, pitch, yaw, heave, surge, and sway. The swing damper compensates for external swing disturbances by introducing slew, luff, and hoist commands that tend to null a pendulation error signal generated by a pendulation sensing mechanism and summed to an internally generated “nominal” pendulation value; it removes energy that has entered the system from external sources (e.g., wind) or from system nonlinearities. The pendulation sensing mechanism must be capable of resolving the position of the payload in a frame of reference fixed to the boom and oriented to the local gravity vector. One means of effecting a solution is via a sensor situated at the upper end of the payload hoist line attached to the boom tip to provide swing angle feedback.
Also of interest regarding PCS are: W. Thomas Zhao and Frank Leban, “Human/Hardware-in-the-Loop Testbed of Cargo Transfer Operations at Sea,” ASNE (American Society of Naval Engineers) Joint Sea Basing Conference, Arlington, Va., Jan. 27-28, 2005, 10 pages, incorporated herein by reference; and, Robinett, III et al. U.S. Pat. No. 6,442,439 B1 issued 27 Aug. 2002, entitled “Pendulation Control System and Method for Rotary Boom Cranes,” incorporated herein by reference. The pendulation control system of Robinett, III et al. '439, which pertains to the command shaping aspect of the Pendulation Control System disclosed by Agostini et al 2002, includes an input command sensor, a pendulation frequency identifier, and a command shaping filter. In a simple type of slewing pedestal crane, the input command sensor responds to the operator commands from the operator input device, and the input commands are thus filtered so as to reduce pendulation. The pendulation frequency identifier indicates the residual payload pendulation frequency of the crane. The command shaping filter filters out the residual payload pendulation frequency from the operator commands.
Other electromechanical and/or algorithmic approaches have been considered for assisting crane operators in controlling slewing pedestal cranes. See, for instance, the following United States patents, each of which is incorporated herein by reference: Nayfeh et al. U.S. Pat. No. 6,631,300 B1 issued 7 Oct. 2003, entitled “Nonlinear Active Control of Dynamical Systems”; Naud et al. U.S. Pat. No. 6,505,574 B1 issued 14 Jan. 2003, entitled “Vertical Motion Compensation for a Crane's Load”; Robinett, III et al. U.S. Pat. No. 6,496,765 B1 issued 17 Dec. 2002, entitled “Control System and Method for Payload Control in Mobile Platform Cranes”; Jacoff et al. U.S. Pat. No. 6,444,486 B2 issued 11 Nov. 2003, entitled “System for Stabilizing and Controlling a Hoisted Load”; Jacoff et al. U.S. Pat. No. 6,439,407 B1 issued 27 August 2002, entitled “System for Stabilizing and Controlling a Hoisted Load”; Overton et al. U.S. Pat. No. 5,961,563 issued 5 Oct. 1999, entitled “Anti-Sway Control for Rotating Boom Cranes”; Robinett, III et al. U.S. Pat. No. 5,908,122 issued 1 Jun. 1999, entitled “Sway Control Method and System for Rotary Boom Cranes”; Nachman et al. U.S. Pat. No. 5,089,972 issued 18 Feb. 1992, entitled “Moored Ship Motion Determination System.” See also, Bonsor et al. United Kingdom Patent Application GB 2267360 A published 12 Jan. 2003, entitled “Method and System for Interacting with Floating Objects,” incorporated herein by reference.
Generally speaking, control systems and methods known in the art for facilitating crane operation are not entirely successful in limiting or alleviating pendulation to acceptable magnitudes under all standard operating conditions.